Recently, as a high-speed signal transmission interface, USB 2.0 standard and IEEE 1394 standard has become prevalent. These interfaces are used in various digital equipment such as personal computers and digital cameras. The USB 2.0 standard and IEEE 1394 standard interfaces adopt a differential signal system in which a pair of data lines is used to transmit a differential signal (differential mode signal), different from a single end transmission system, which has been generally used heretofore.
The differential transmission system has excellent characteristics such that a radiation electromagnetic field generated from the data line is less than that in the single end transmission system and it is hardly affected by exogenous noise. Therefore, it is easy to minimize the amplitude of the signal, and signal transmission can be performed at a higher speed than the single end transmission system by reducing rise time and fall time because of the small amplitude.
FIG. 6 is a circuit diagram of a conventional differential transmission circuit.
The differential transmission circuit shown in FIG. 6 includes a pair of data lines 2 and 4, an output buffer 6 that supplies a differential mode signal to the pair of data lines 2 and 4, and an input buffer 8 that receives the differential mode signal from the pair of data lines 2 and 4. According to such a configuration, an input signal IN to be provided to the output buffer 6 is transmitted to the input buffer 8 via the data lines 2 and 4, and reproduced as an output signal OUT. The differential transmission circuit has such a characteristic that the radiation electromagnetic field generated from the data lines 2 and 4 is less. However, when common noise (common mode noise) is superimposed on the data lines 2 and 4, a relatively large radiation electromagnetic field is generated. In order to reduce the radiation electromagnetic field generated by the common mode noise, as shown in FIG. 6, it is effective to insert a common mode choke coil 10 in the data lines 2 and 4.
The common mode choke coil 10 has such characteristics that an impedance with respect to a differential component (differential mode signal) transmitted on the data lines 2 and 4 is low, and an impedance with respect to an in-phase component (common mode noise) is high. Therefore, by inserting the common mode choke coil 10 in the data lines 2 and 4, the common mode noise transmitted on the pair of data lines 2 and 4 can be intercepted without substantially attenuating the differential mode signal. There has been known a laminated common mode choke coil described in, for example, Japanese Patent Application Laid-open Nos. H8-203737, 2005-12071, and 2005-12072.
The common mode choke coil has the characteristic that the differential mode signal is not attenuated substantially. However, as the frequency becomes high, the attenuation of the differential mode signal gradually increases. Accordingly, to increase the signal quality in the high-speed interface, the cut-off frequency with respect to the differential mode signal needs to be increased further.